1. Field of the Invention
The invention is in the field of voltage surge protection devices, such as are used to protect telephone station apparatus from external voltage surges (e.g., lightning strikes and accidental contact between telephone lines and power lines).
2. Description of the Prior Art
In transmission systems with large stretches of outdoor wiring, it is common to protect terminal equipment from voltage surges (e.g., lightning strikes) by the inclusion of a protective device between the line and ground at each terminal. Such devices should be capable of sustaining repeated voltage surges without failing but when they fail, they should fail to an electrically short circuit condition (fail safe). A widely used class of surge protective devices includes two carbon block electrodes with parallel faces defining an air gap of the order of 0.1 millimeter. This is an extremely inexpensive device, however, the labor cost of replacing failed devices in the field is high. Thus, efforts have been continuously made to extend the service life of such devices, even at the expense of somewhat higher cost. One modification which has been developed is the inclusion of grooves in the carbon block face to accept debris formed during the protective breakdown. (U.S. Pat. No. 3,703,665 issued Nov. 21, 1972.) Another class of devices seeks to prolong service life through the use of carbon-coated metal electrodes sealed in a protective environment. Such devices have been made with setbacks in the electrodes to suppress failure produced by the sputtering of conductive material during breakdown (see, for example, U.S. Pat. No. 3,898,533 issued Aug. 5, 1975). These sealed devices are significantly more expensive than carbon block devices. However, when the factors such as the labor cost for replacement of failed devices is taken into account, their use is often indicated. A metal electrode protector with protrusions in the gap faces to avoid pitting is disclosed in U.S. Pat. No. 3,119,040 issued Jan. 21, 1964 and a protector with metal electrodes with a central portion of a high resistance (megohms) semiconductor material (e.g., silicon carbide) is disclosed in U.S. Pat. No. 2,923,849, issued Feb. 1, 1960.
In a recently developed approach to this problem (U.S. Pat. No. 4,020,398 issued Apr. 26, 1977 to L. G. McKnight), the electrodes bound a narrow gap region, to set the protective breakdown voltage, and a wider gap region, which will sustain the major part of the electrode damage. Shortly after the initiation of the protective discharge in the narrow gap region, the discharge is forced into the wider gap region by the provision, in at least one of the electrodes, of a high resistance path at the narrow gap region. Since the major portion of electrode damage is sustained by the wider gap region of the electrodes, the incidence of shorting failure is suppressed.
For use in many situations it is a further requirement placed on these devices, that they be capable of maintaining repeated low current or low energy surges without failing "open" by erosion of the material at the narrow gap region, producing an unacceptably high breakdown voltage. The particular electrode geometries disclosed in U.S. Pat. No. 4,020,398 showed some tendency toward this fault, leading to a search for an improved electrode structure.